1. Field of the Invention
The present invention relates to a polypropylene resin composition. More specifically, it relates to a polypropylene resin composition having a superior fluidity of the molten resin when injection molding and, capable of providing a shaped article having a high rigidity, high low-temperature impact resistance, and superior heat resistance, and suitable for use as an auto part or other structural member.
2. Description of the Related Art
A polypropylene resin is easy to mold and, among conventional resins, has a high melting point, and therefore, has a superior heat resistance and further light weight. Thus, a polypropylene resin is widely used in the numerous industrial fields such as automobiles, electrical parts. However, a polypropylene resin is not necessarily sufficient, compared with other conventional resins, in the applications where a high rigidity and low-temperature impact resistance are simultaneously demanded.
The low-temperature impact resistance of a polypropylene resin becomes inferior due to the fact that the secondary transition point of polypropylene is around 0.degree. C. That is, at a temperature of less than 0.degree. C., the motion of the polypropylene molecular chains freezes, which results in a glass state, and therefore, the energy of external impact cannot be absorbed, whereby breakage occurs.
Therefore, as a means for improving the low-temperature impact resistance of polypropylene, in the past a polymer having rubbery properties and a secondary transition point of less than approximately -50.degree. C. is mixed into the polypropylene. Examples of such a polymer having rubbery properties are ethylene-.alpha.-olefin copolymers, which are generally amorphous and low in crystallinity.
For example, Japanese Examined Patent Publication (Kokoku) No. 58-25693 and Japanese Examined Patent Publication (Kokoku) No. 58-38459 disclose compositions including crystalline polypropylene and an ethylene-butene-1 copolymer comprising not more than 15 molar % of butene-1. However, the resultant compositions have low impact resistance at low temperatures. Further, Japanese Examined Patent Publication (Kokoku) No. 63-42929 discloses a polypropylene composition containing an ethylene-butene-1 copolymer. However, in this case as well while the impact resistance is improved, the heat resistance is not sufficient.
When a rubbery substance is added to a polypropylene resin as explained above, the low-temperature impact resistance is improved with the increase in the amount of the rubbery substance added. However, when a large amount of the rubbery substance is included, the rigidity, heat deformation temperature, and surface hardness of the resultant composition are all largely decreased.
Therefore, there are limits to the addition of the rubbery substance. Methods for the simultaneous addition of talc or another inorganic filler are known so as to maintain the rigidity, but there are also limits to the amount of addition of the filler. Namely, when a large amount of the filler is added, the toughness is lost and the effect of addition of the rubber, that is, the impact resistance, is also impaired, and therefore, it has been difficult to use such a composition for applications where a high rigidity and ordinary-temperature and low-temperature impact resistance are simultaneously required.
Further, for example, polypropylene compositions containing a rubbery substance and inorganic fillers such as talc have been proposed (see Japanese Unexamined Patent Publication (Kokai) No. 60-58459, Japanese Unexamined Patent Publication (Kokai) No. 60-60154, Japanese Unexamined Patent Publication (Kokai) No. 62-235350, Japanese Unexamined Patent Publication (Kokai) No. 63-122751, Japanese Unexamined Patent Publication (Kokai) No. 1-149845, Japanese Unexamined Patent Publication (Kokai) No. 1-204947, Japanese Unexamined Patent Publication (Kokai) No. 1-271450, etc.). However, these compositions also have little effect in simultaneously achieving both rigidity and impact resistance.